Method for low temperature electrosugery and rf generator

ABSTRACT

Method and apparatus for electrosurgery including tissue coagulation using very high voltage pulses of electrical energy applied to the electrosurgical probe. This minimizes heating of the surrounding tissue in the probe and is especially suitable for precise and limited coagulation and fulguration without excessive tissue charring or other damage. The power at rated load of the applied pulses to the probe is typically over 300 W and the duration of the on time is very short, so each group of pulse bursts is of relatively low duty cycle. An RF generator is also provided for delivering electrical energy to an electrosurgical probe with the proper characteristics, including fast switching times.

FIELD OF THE INVENTION

This invention relates to electrosurgery for biological tissue.

BACKGROUND OF THE INVENTION

The field of electrosurgery is well known, see for instance, PalankerU.S. Pat. No. 7,238,185 and Palanker, et al. U.S. Pat. No. 6,780,178both incorporated herein in their entireties. Briefly, application of avoltage to an electrode is useful for cutting, ablating and fulguratingbiological tissue. This is generally known as electrosurgery. Typicallythe voltage is applied as a train of high frequency pulses in the radiofrequency (RF) range to a probe in contact with the tissue.

A problem with electrosurgery is preventing excessive application ofheat to the tissue being cut, fulgurated, dessicated, etc. since thistends to produce undesirable affects such as charring and collateraltissue damage. This is typically caused by high temperatures induced bythe application of the electrical energy.

Some highly localized high temperature is required during, for instance,tissue coagulation (sealing) for denaturation of blood and vasculartissue (veins and arteries) followed by occlusions of the blood vessels.Typically dessication occurs below or close to 100° C. and fulgurationat higher temperatures above 100° C. A high temperature duringfulguration outside the immediate area being treated results inundesirable tissue charring and buildup of debris on the electrosurgicalprobe, which decreases its efficiency of coagulation. This may alsoresult in adhesion of charred tissue to the probe and damage to theareas of the probe with low melting temperatures such as plasticcomponents. Typically this might require cleaning of the probe aftereach session of coagulation. Also, high temperature may result in smokeobscuring the surgical field, especially for laparoscopic procedures.

SUMMARY

In accordance with this invention, a method and apparatus for pulsedapplications of heat in electrosurgery provide sufficient peaktemperature for tissue coagulation (and “blend” cutting) and allow forcooling of tissue between the application of electrical energy pulses,so avoiding excessive heating. Typically groups of pulse bursts areseparated by a time interval sufficient for cooling both the probe andimmediately neighboring tissue to close to ambient temperature.

In one embodiment this is achieved by using RF high power groups ofpulse bursts, such as power levels of 300 W or higher during on time andzero during off time, the groups of bursts being of high frequency suchas 100 kHz to 5000 kHz and each group of bursts having a duty cycle inthe range of 1% to 50%. Duty cycle refers to the ratio of time when RFpower is applied to the rated load to the full duration of the group ofbursts. According to this definition a sine wave has a 100% duty cycle.For coagulation and blend cutting electrosurgery, the sine wave cycles(on time) occupy a short time in each burst, with a substantial part ofeach burst having no RF energy present (off time). A number of suchbursts are grouped together, with an interval of at least 1 millisecondbetween each group of bursts, to allow for tissue cooling. Each burst ofpulses has enough electrical power to rapidly heat the tissue totemperatures adequate for coagulation.

The active portion of the electrosurgery probe itself is typically ofrelatively small size to provide a short cooling time. Moreover theprobe is bare metal or metal covered with a layer of insulation, withthe layer of insulation defining an opening at the edge where theelectrical pulses are actually applied to the tissue for coagulation andcutting and further defining a number of spaced apart small openings onits side surfaces (flat portions), each having a diameter for instanceof 0.02 mm to 0.10 mm for extensive coagulation.

Also provided in accordance with the invention is an RF pulse generatorfor low temperature electrosurgery tissue cutting. This pulse generatorprovides square wave alternating positive and negative pulses with afast switching time and a pulse amplitude of up to 1000 Volts peak topeak. The particular circuit disclosed here, also referred to as a pulsegenerator or radio frequency generator in the field, is based on aconventional half bridge inverter with high power transistors serving ashigh and low side switches. In order to overcome the well known problemof Miller capacitance, each channel of the inverter (there being apositive pulse voltage channel and a negative pulse voltage channel) isprovided with a gate driver circuit driving the gate of each switchingtransistor. Moreover an input terminal of the gate driving circuit isnegative biased and also coupled to ground via a resistance. Further,each channel also includes a current driver (booster) with a disablefunction to provide protection of the circuit in short circuitconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a, 1 b, 1 c show a set of high power groups of bursts indicatingthe nature of the electrical energy applied to the probe in accordancewith the invention for 3 types of coagulation respectively spray,pinpoint, and blend.

FIG. 2 shows a graph of thermal relaxation time vs. probe size.

FIG. 3 is a planar view of a portion of an electrosurgical probe showingthe openings defined on the side surfaces of the probe through theinsulation layer.

FIG. 4 a is a schematic circuit diagram of a RF generator in accordancewith the invention.

FIG. 4 b shows an output waveform of the RF generator.

FIG. 5 a shows the potential problem of noise in the present RFgenerator.

FIG. 5 b shows how the problem of noise is overcome in accordance withthe present RF generator.

FIG. 6 shows a schematic diagram of a protection circuit used with theRF generator of FIG. 4 a.

DETAILED DESCRIPTION High Voltage Electrosurgery

The present description is directed to high voltage RF electrical energyapplied to an electrosurgical probe for tissue coagulation and cutting.It is understood that the electrosurgery probe itself may be of thetypes disclosed here or in the above described patent applications orother types as known in the field. Typically the probe has a relativelysmall surface area at its active electrode portion (tip) to minimizeheating of the tissue being treated. The probe may be uninsulated (baremetal) or partly covered with a high dielectric insulating layer. Theprobe may be mono or bi-polar. In some applications the probe isimmersed in the tissue being operated on, which has naturally occurringfluid present or some type of liquid is provided immediately around theprobe in the surgical field. In other uses for, e.g., fulguration noliquid is present.

The present method is intended primarily for use with electrosurgicalcoagulation, but can be used for simultaneous tissue cutting andcoagulation. For tissue coagulation purposes some amount of charring isin fact desirable since that is the intent of coagulation (to sealtissue). However, tissue charring is undesirable beyond the immediatearea being coagulated. The goal is to maintain a relatively low probetemperature and hence minimize heat transfer to the surrounding tissuewhile still accomplishing coagulation or dessication or fulguration.Hence the present method is directed to what is sometimes called “coldcoagulation.”

The edge of the probe is intended for both cutting and coagulation, andthe flat (side) portion thereof with “dimples” (the openings) serves forcoagulation only. As the probe edge cuts through the tissue, the flatportion of the probe sends an electrical arc to the walls of the woundto heat and close the blood vessels. The dimples help the electrical arcto reach all blood vessels, as in an uninsulated probe, but the smalldiameter of the dimples advantageously provides a short thermalrelaxation time and, as a result, low temperature during pulsedcoagulation.

This is accomplished here by applying high power RF groups of burstswith relatively long off times and thus relatively low duty cyclescompared to conventional electrosurgical coagulation. While equipmentlimitations may prevent use of RF power levels above 300 W given currentmaterials and electrical components, ultimately this is not limiting.Hence generally, the present invention is directed to use of high ratedpower RF (above 300 W) during the pulse on time. Since power depends onthe load, the rated load is by definition the load where the maximum(rated) power can be achieved. A typical voltage here (both positive andnegative) is up to 12,000 Volts peak to peak under open circuitconditions. A typical waveform for this condition is a damped sine wave.

In one embodiment the RF power has a carrier frequency of approximately460 KHz, so the duration of each period (pulse) is approximately 2.2microseconds. The on time RF pulses can be sine waves, but usually asine wave is good only for pure cutting. For blended cut and coagulationpurposes, periods of pulses are clustered in each burst with no RFenergy between them. In one embodiment, there is only one period perburst but this is not limiting; there may be 2, 3, or more pulses perburst as shown respectively in FIGS. 1 a, 1 b, 1 c for different typesof coagulation. The repetition rate for the bursts is e.g. 30 KHz. Atypical frequency for the groups of burst is 25 Hz. A group of thesebursts defines the on time, followed by the off time. Hence the dutycycle of each group (during on time) is in the range of 1% to 50%. Thatis, only 1% to 50% of the total time during each group of bursts isactually occupied by RF energy and the remainder is of zero voltageapplied to the load, as shown in FIGS. 1, 1 a, 1 b, 1 c. A number ofsuch pulse bursts may also be grouped together. Typically the off timebetween each group of bursts is about 1 millisecond or more to allowfurther cooling of the probe and associated tissue. The on time can befrom 100 microseconds to 10 ms, followed by the off time off interval,at least 1 ms in duration. This modulation further reduces the dutycycle by a factor of 0.01 to 0.9.

The open circuit waveform is, e.g., a damped sine wave at the carrierfrequency (such as 460 kHz in FIG. 1 c) which shrinks to a sine wavecycle as shown in FIG. 1 a at a low impedance load. The amplitude(voltage) of these pulses decreases with the resistance of the load insuch a way that the average RF power achieves a maximum at a so called“rated load”, typically 100 to 1000 Ohm. The time T between pulsecenters corresponds to the inverse of the carrier frequency (for exampleT=1/460 kHz). The number of pulses in a pulse burst determines the ratedload, roll-off points on the load curve, the type of the surgical mode(called in the field for instance blend, desiccation, fulguration), andthe length of the spark. The repetition rate of the pulse bursts istypically 20 to 60 kHz (indicated as fburst of 30 kHz in FIG. 1 b). Thepurpose of these bursts is not a cooling of the tissue during the burstoff time, since tissue temperature cannot decrease during mere tens ofmicroseconds. Instead the off time allows for collapse of undesirablevapor bubbles formed on or near the probe, arising during the pulse ontime. Otherwise due to the vapor bubbles the tissue experiencesproblematic “micro explosions” of the bubbles, repulsing tissue from theprobe, and precluding effective coagulation.

In the present method therefore for coagulation the pulse bursts aregrouped together, with a time interval between them (determined by theburst duty cycle) longer than the thermal relaxation time for aparticular probe. The thermal relaxation time t can be assessed ast=r²cρ/π²k=r²0.7 μs/μm², where r is the characteristic size of theelectrode in μm, c is heat capacity, ρ is density, and k is thermalconductivity of liquid as plotted in FIG. 2. For instance, for a 10 μmeffective probe electrode size the thermal relaxation time is 70 μs, butfor 1 mm probe this value is 0.7 seconds. So, a small probe electrode isgenerally required here.

The small probe can coagulate only a small area adjacent to the probeelectrode. With a spark (arc) length of 1 mm and a point electrode thatis 0.1 mm in diameter, one can coagulate a spot of tissue 2.1 mm indiameter. Multiple small electrodes, representing small openings in theprobe insulation are introduced on the flat portion of the blade andspaced apart to coagulate a large solid area of tissue. The sparkcircles should overlap to cover the whole tissue surface. The size ofthe individual electrodes (the openings) is small enough to provide fastcooling. At pulsed mode as described above, low average tissuetemperature can be achieved. As a result, the probe provides shallowstrong coagulation with a safe temperature of the probe.

FIG. 3 shows a partial view of a side surface of the associated probe 30in which the overlying insulating layer 34 defines a pattern of openings(dimples) 36 a, 36 b, 36 c, etc. to expose the underlying metal 40 ofthe electrode of probe 30. In this case the size of each opening isshown as about 0.05 mm by 0.05 mm (each being approximately a square),however, this size and shape are not limiting. The spacing betweenopenings (center to center) is in the range of 0.2 to 1.0 mm, notlimiting. Also conventionally the metal edge 42 of probe 30 is alsoexposed through the overlying insulating layer 34. The actual materialsof the probe metal 40 and insulation 34 are conventional as explained inthe above referenced patents and as well known in the field.

Typically the associated equipment (RF generator) has variable outputsand can be adjusted by the operator to provide pulses of variousfrequencies and timing durations so that the present pulse regime isthereby accomplished. This pulse regime may depend on probe size, thenature of the surgical procedure being undertaken such as fulguration,dessication, coagulation, and other factors as determined by theoperator (surgeon). A typical range of correct frequencies for thepulses is 100 KHz to 5 MHz, of which the above described 460 KHz ismerely illustrative. In accordance with this approach, the probe and theassociated tissue may be kept below or above 100° C., depending on whatis required for the particular surgical procedure being undertaken.Advantageously the relatively low temperature of the probe-tissueinterface results in reduced adhesion of the charred tissue to theprobe, decreasing smoke and providing better performance forcoagulation.

RF Generator

Also disclosed here is an RF generator 50 for e.g. pulsed cutting oftissue (see circuit diagram FIG. 4 a), compatible with a pulsedcoagulation method and probe described above. Such an RF generator isbelieved to be novel for electrosurgery, where out coupling typicallyrepresents a transformer, although generally RF generators are wellknown in the electronics field for generating high frequency electricalsignals. Such RF generators typically are half bridge inverters. Thepresent RF generator has only capacitors in series with the load asrequired by regulatory rules (for a capacitance <5 nF), and the usual RFtransformer is omitted. Associated waveforms are described in PalankerU.S. Pat. No. 7,238,185, incorporated herein by reference in itsentirety, and represent a true bipolar square wave. The present RFgenerator for tissue cutting may be a part of a system producing alsocoagulation waveforms according to FIG. 1 to combine cutting andcoagulation ability for a single probe.

In accordance with similar circuits, the present RF generator apparatusor circuit 50 conventionally includes a half bridge inverter with highpower Field Effect Transistors (FET) Q2, Q1 used as respective low andhigh side switches. In such an RF switching generator or power supplythe amount of time for each switch (transistors Q1, Q2) to turn on oroff is important. For proper performance the switching transistorsshould be capable of switching in less than approximately 10% of theperiod of the output pulse. For a 4 MHz frequency pulse this requiresthat each transistor's Q1, Q2 gate terminal be charged/discharged inless than 25 nanoseconds.

As well known in the field, the effective gate capacitance or inputcapacitance of such field effect transistors Q1, Q2 includes thegate-source capacitance and the gate-drain capacitance, also referred toas Miller capacitance. The total gate charge required to charge the gateof a typical field effect transistor from 0V to 3V (enough to switch thetransistor) is 80 nC. This total charge includes the Miller chargerequired to discharge the gate-drain capacitance when the transistorswitches from the off state with a drain-source voltage of 450V, to theon state. If the entire charge is to be delivered in a 25 nanosecondperiod as indicated above, then the gate driver circuit which providesthe signal to the gate must apply an average current of 80 nC/25 ns=3.2amps with a peak current as high as 12 amps. To meet this, the gatedriver circuits 52, 54 in this generator are selected to provide a 20amp maximum current, but this is merely illustrative.

For typical electrosurgery applications, the electrical charge injectedin the tissue from the probe must be close to zero to minimizeundesirable muscle stimulation. Thus it is important to have balancedpositive and negative portions of the pulsed current provided to theprobe. In the present RF generator therefore two channels 58, 60 areused, e.g., connected to two Direct Current (DC) power supplies (notshown), one providing +500V and the second providing −500V outputsignals at respectively nodes 115, 117. The output terminal 112 (to theprobe) for generator 50 therefore is connected at a midpoint node 66between the two channels 58, 60. Voltage at this terminal 66 thereforeswings between positive and negative voltages as described furtherbelow.

The current driving for the gate of each switching transistor Q1, Q2 isprovided here with a radio frequency isolated independent driving directcurrent power supply. The RF isolation is required because the gate ofeach transistor Q1, Q2 is referenced to the source of the transistor,which switches with slew rates of more than 30 Volt/nanosecond. The highside driving reference point 66 has to have a minimum couplingcapacitance and leakage inductance to the ground of the drivers 52, 54.

Transformers 78, 80 are provided as is conventional in each channel 58,60 for galvanic isolation and level shifting required for each switchingtransistor Q1, Q2. An advantage of this is at the high-side gate drivercircuit 52 does not require a floating power supply since the power totransistor Q1 is coupled through the transformer 78. The leakageinductance of the windings of each transformer 78, 80 makes it difficultto obtain the rapid rise of the current required and causes excessiveringing which must be suppressed. Improved operation is obtained here byusing a large sinusoidal drive current since the leakage inductance ofthe transformers 78, 80 along with the input capacitance of eachtransistor Q1, Q2 can be included in a resonant circuit.

In this case the sine wave output of the resonant circuit has higheramplitude than the transistor switching threshold voltage, to minimizeswitching time. However, fast switching results in shorter high voltageswings on the source/drain terminals of the switching transistors Q1,Q2. Short intense transients therefore travel back from the mid-point 66of the half bridge to the gate terminals of transistors Q1, Q2 due tothe Miller capacitance into the output of each gate driver circuit 52,54 from each of the transformers 78, 80. Each gate driver circuit 52, 54has a 0.6 Ohm output resistance in both high and low output voltageregimes. Therefore the energy of the transient goes mostly to the lowvoltage ground as indicated in FIG. 5 a and causes ringing. Thisundesirable ringing may affect the input of the gate driver circuits 52,54 causing simultaneously opening and closure of the switchingtransistors Q1, Q2 which, of course, must be avoided. In order toincrease signal to noise discrimination level and avoid ringing, anegative bias voltage as shown of −1V is applied to the input (“In”)terminal of each gate driver circuit 52, 54. Additionally in this casethe same input terminal of each gate driver circuit 52, 54 is alsocoupled to ground via a low resistance (22 Ohm) resistor 80, 82. Also,22 Ohm resistors 86, 88 are coupled across the primary and/or secondarywindings of each transformer 78, 80 to damp ringing. Also, to decreasequality factor of the resonant circuit, inductances of the primary andsecondary windings of each transformer 78, 80 are chosen to be minimale.g. 1.6 microH. With the input capacitance of the MOSFET Q1, Q2 (1 nF)resonant frequency of the contour f=1/(2π(LC)^(0.5))=4 MHz is equal tothe operation frequency. The inductance of the ground path to the inputterminal of each gate driver circuit 52, 54 is minimized with short andwide leads. Also, a DC/DC converter 90, 92 is coupled to the inputterminal of each gate driver circuit 52, 54 to create the abovementioned negative direct current bias of −1 Volt to that input terminaland effectively discriminate noise at the input terminal of each gatedriver circuit 52, 54.

As shown in FIG. 4 a, effectively the negative input bias at the inputterminal “In” of each gate driver circuit 52, 54 is −1 Volt in thisexample. In the left hand portion of FIG. 4 a are shown (as waveforms)the input control signals 100, 102 applied to each input terminal 106,108 of the two channels of the RF generator and shown as a set of squarewaves which determines timing for the pulse bursts and pulses asexplained above. The input control signals are generated conventionally.Conventionally in the far right hand portion of FIG. 4 a is the RFgenerator output terminal 112 labeled “pulse out” which is connected tothe probe. FIG. 4 b shows an output waveform (at node 112) of the RFgenerator 50. Also provided is a high voltage ground terminal 116connected to the probe ground terminal or to a return line connected tothe patient. The remaining circuit elements in FIG. 4 a areconventional; in some cases component numbers or values are shown, butthese are only exemplary.

FIG. 5 a shows via waveforms how the circuit of FIG. 4 would have anaccumulation of noise on a low voltage ground resulting in anuncontrollable wave form, e.g., due to ringing. The horizontal axis hererefers to time and the vertical axis is the voltage at the inputterminal of each gate driver. The horizontal broken line at 3 Volt isthe threshold voltage at the input terminal of the gate drivers 52, 54.FIG. 4 b shows how the above described negative bias of −1 Volt appliedto that same input terminal (and also shown as the horizontal brokenline in FIG. 5 b) reduces the amount of noise compared to FIG. 4 a atthe In terminal to the gate driver circuits 52, 54.

Also provided in one embodiment in the circuit of FIG. 4 a is overcurrent protection to prevent damage to the switching transistors and/orthe other components. Typically failure of such a RF generator is causedby excessive currents flowing either through the switching transistorsor into the output terminal. Various conventional protection circuitsare known and an example is shown in FIG. 6 which would be coupledconventionally to generator 50. These protection circuits typicallyinclude current transformer sensors connected either to the returnpatient cable (ground to the patient, e.g., at node 116) or to the highvoltage lines at node 112. The circuit of FIG. 4 a since it has twochannels 58, 60 would typically have two such protection circuits, onecoupled to each channel 58, 60.

This description is illustrative and not limiting. Further modificationsand improvements will be apparent to those skilled in the art in lightof this disclosure and are intended to fall within the scope of thepending claims.

1. A method of coagulating tissue, comprising the acts of: applyinggroups of bursts of pulsed electrical energy to a probe in contact withthe tissue; each group of bursts having a power of at least 300 W at arated load during an on time and zero during an off time, the RF groupsof bursts being of frequency in a range of 100 kHz to 5000 kHz and eachgroup of bursts having a duty cycle in a range of 1% to 50%; applying aplurality of the bursts in succession; and providing an interval betweensuccessive pluralities of the bursts of at least 1 msec.
 2. The methodof claim 1, wherein a temperature of the probe remains below 100° C. 3.The method of claim 1, wherein a tip of the probe is of metal with aninsulation layer thereover, the insulation defining on a side of theprobe a plurality of openings to expose the metal.
 4. The method ofclaim 3, wherein each opening has a diameter in the range of 0.02 mm to0.010 mm.
 5. The method of claim 3, wherein at an edge of the probe theinsulation defines an opening to expose the metal.
 6. Electrosurgeryapparatus, comprising: a probe adapted to be applied to tissue; and asource of electrical energy electrically coupled to the probe, thesource applying groups of bursts of pulsed electrical energy to theprobe; each group of bursts having a power of at least 300 W at ratedload during an on time and zero during an off time, the RF groups ofbursts being of frequency in a range of 100 kHz to 5000 kHz and eachgroup of bursts having a duty cycle in a range of 1% to 50%; the sourceapplying a plurality of the bursts in succession; and the sourceproviding an interval between successive pluralities of the bursts of atleast 1 msec.
 7. A circuit to generate high frequency signals,comprising: a first channel and a second channel, each including aswitching transistor, an output terminal of each transistor beingcoupled to a common output node; a control terminal of each transistorbeing coupled to an output terminal of a driver; and an input terminalof each driver being coupled to a source of a negative voltage bias;wherein the first channel provides positive going signals at the commonoutput node and the second channel provides negative going signals atthe common output node.
 8. The circuit of claim 7, further comprisingcoupling the input terminal of each driver to ground.
 9. The circuit ofclaim 8, further comprising: a current driver circuit having an outputterminal coupled to the input terminal of each driver.
 10. The circuitof claim 8, further comprising a current protection circuit coupled toeach channel.